1. Field of the Invention
This invention relates to uses of polyvinyl alcohol copolymers.
2. Description of the Prior Art
There is a consumer need for water-soluble packaging, which offers the consumer convenience and can reduce packaging waste persistent in the environment when the packaging material is ultimately degraded by microorganisms at sewage treatment plants. This convenience to the consumer can also include the benefit of unit-dose packaging, which does not require measurement or handling of package contents. It can be particularly attractive to package laundry detergent in such a manner, and in fact such packaging is in use today to deliver liquid laundry detergent and automatic dishwasher cleaning agents.
Water-soluble packaging material commonly used for liquid laundry detergent is often based on polyvinyl alcohol (PVOH) films. PVOH is manufactured commercially by polymerization of vinyl acetate monomer (VAM) to afford polyvinyl acetate (PVAc). The PVAc is then transesterified—in most commercial processes with methanol—to afford PVOH and methyl acetate. PVOH that is >98% hydrolyzed (that is, less than 2% residual vinyl acetate) is unacceptable for this application because it does not dissolve at practical temperatures, usually requiring temperatures in excess of 50° C. to dissolve. This temperature is inconvenient for consumers. Of greater utility to consumers are packaging materials that dissolve in cold water, that is, water at a temperature of from 10-25° C. This can be accomplished by modifying PVOH to reduce crystallinity and/or increase hydrophilicity of the polymer. The crystallinity of the polymer is conveniently and conventionally reduced by carrying out the transesterification of PVAc in such a manner as to not complete the conversion to PVOH and obtain a product that is conventionally known as partially hydrolyzed PVOH (phPVOH). Commercial grades of phPVOH include Celvol 523 from Celanese Chemicals and Kuraray POVAL PVA 217 sold by Kuraray Co., Ltd. The degree of conversion (loosely referred in the industry as “hydrolysis”) in most cases varies from 78-99.8%, and 88% hydrolyzed is an especially common grade for water soluble packaging. Post-polymerization or post-copolymerization modifications of PVOH can reduce crystallinity. Adding polar groups to the polymer can increase the hydrophilicity of the polymer. This can be accomplished by post-polymerization reaction or by copolymerization method. Post polymerization reactions have been reviewed (Polyvinyl Alcohol-Developments; Finch, C. A., Ed.; John Wiley & Sons: New York, 1992). One known copolymerization method to increase hydrophilicity is to copolymerize VAM with an acid-containing monomer such as acrylic acid or others as described in U.S. Pat. No. 4,885,105. After transesterification of such a copolymer a hydrophilic carboxylic acid or carboxylic acid salt remains. Inclusion of up to 10 mol % comonomer can modify the film solubility adequately to satisfy consumers. A cold water soluble film can be obtained by copolymerization of VAM with a monomer that later undergoes a chemical reaction to render it hydrophilic. For example, VAM/methyl acrylate (MA) copolymer is known to undergo transesterification reaction to a PVOH copolymer where the MA carboxyl group has formed a lactone structure with a neighboring alcohol unit (Polyvinyl Alcohol-Developments; Finch, C. A., Ed.; John Wiley & Sons: New York, 1992). This lactone-containing polymer is not soluble in cold water. However, after treatment with alkali such as sodium hydroxide the lactone is converted to the ring-opened, sodium carboxylate form which is cold water soluble.
In order to be practical and useful, the films should be compatible with the chemicals they contain, at least for enough time that premature rupture of the package does not occur. For example, a film made from phPVOH is sensitive to high or low pH chemicals, as they cause hydrolysis of the residual vinyl acetate, effecting change to cold-water insoluble PVOH.
PVOH copolymers from monomers containing carboxylate or carboxylate precursor groups are stable to high pH. However at lower pH the carboxylic acid can undergo ion-exchange with protons in the film's encapsulate. If the number of carbon atoms separating the carboxylate carbon and oxygen atom of a neighboring alcohol group is equal to three, then the groups can undergo lactonization to afford the water-insoluble lactone-containing copolymer. This package then does not completely dissolve, leading to consumer dissatisfaction. The pH where lactonization reaction begins to occur is about <8, depending on the concentration of the ions in the encapsulate.
It has been found that a copolymer that includes itaconic acid (IA), which is convenient to manufacture due to its high conversion during polymerization, can be a useful carboxylate copolymer. WO 94/04656 describes use of 2-8 mol % IA copolymer in water soluble packaging application. This copolymer retains cold water solubility to a better extent than a methyl acrylate copolymer that has been converted to sodium carboxylate form by caustic. For example, a film using greater than or equal to 4 mol % IA gave good solubility stability after prolonged contact with Ariel liquid laundry detergent. However a film with 2.8% IA had inadequate solubility stability, surprising in view of the fact that WO 94/04656 teaches that 2-8 mol % IA copolymer has desirable solubility properties. However, the IA monomer has some disadvantages. It is a higher cost monomer than VAM. Also most commercial transesterification processes utilize a basic catalyst. The IA in the copolymer is neutralized in an acid-base reaction with the basic catalyst, which adds cost to the process. The increased catalyst demand can also exceed the feeding capability of existing facilities, necessitating costly modifications. Thus it can be desirable to manufacture an IA copolymer having good cold water solubility with the lowest possible IA content.